Many electronic devices include some audio functionality, for instance for media playback or voice communication and/or voice control. Increasing audio data may be stored in a digital format and audio signals may be transmitted and processed as digital signals, however there are still a number of functions that may commonly use analogue audio signals. For instance analogue audio signals may be used to drive audio output transducers, e.g. loudspeakers.
Many electronic devices may thus include an audio signal path that receives an input digital audio signal and which converts the digital audio signal to an analogue audio signal. The analogue audio signal may be processed and may be propagated along an analogue part of the signal path, for instance to an audio load such as a loudspeaker. For example, an analogue audio driving signal may be supplied to an audio load of the host device or output to an external peripheral device, which may be removably connected to the host device via suitable connectors, e.g. a plug and socket. The external device could be a peripheral device such as a headset with relatively small loudspeakers, e.g. located in earphones or earbuds or the like. Such headset loudspeakers will typically have a relatively low impedance, say of the order of a few ohms or so.
There is a trend for increasing the audio performance from electronic devices, especially portable electronic devices. However for such devices size and power consumption are also deemed to be important considerations and reducing the size and power may often conflict with increasing audio performance.
For example, considering distortion, various factors may contribute to distortion in an audio signal path such as a signal path for an amplifier circuit driving a load. Typically the distortion is worst for lower impedance loads. An amplifier may be designed to maintain adequately low distortion into a low impedance load by increasing the output stage quiescent current, generally requiring larger output driving devices and higher current pre-driver stages. Distortion may also be managed by designing the amplifier circuitry with multiple amplifier stages to attempt to suppress output distortion by a high open-loop amplifier gain. However these approaches generally increase both the power consumption of the amplifier circuitry and the size and complexity, and hence cost, of the circuitry.